Sync ako/mxcli: rewrites stop deleting what the statement did not restate, authorable message definitions, and a project knowledge store

[ako]

Thirty-nine commits since the last sync. Two themes dominate: a new project
knowledge store, and a run of fixes to things that a rewrite silently
deleted — documentation, mapping samples, a SOAP binding.

A rewrite no longer deletes what the statement did not restate

Four separate cases, all the same shape: create or replace rebuilds a document
from the statement, so anything the statement does not mention is gone. None of
them is a build error, which is why they survived.

• Documentation (CREATE OR REPLACE / CREATE OR MODIFY deletes an object's doc comment (Documentation), silently #1018). create or modify set Documentation from the
  statement unconditionally, so a statement with no /** … */ wrote the zero
  value over the stored one. Not a drop — an empty overwrite. Absent and
  empty are different facts and only the parser knows which, so
  findDocCommentText now distinguishes them. Carried across a full pass of
  all 29 doctypes.
• A mapping's OriginalValue — the sample parsed from the JSON structure's
  snippet. The old behaviour rested on a measurement over two mappings a blank
  app ships; at corpus scale 2,322 of 3,042 elements carry one. But neither
  default is right: the split is per document (145 mappings all, 107 none, 2
  mixed), so a rewrite carries the stored value forward instead of choosing. The
  original decision stands for newly authored mappings.
• A SOAP mapping's web-service binding. A mapping has four schema sources,
  not three; the fourth was absent from the semantic type, so every rewrite
  destroyed it (CE6896 + CE0270). Now refused rather than rewritten —
  carrying it would claim the rest of the document survives too, which mxcli
  cannot check.
• A JSON structure's snippet formatting. describe pretty-prints, so
  describe → exec reformatted a one-line snippet. Now preserved when the JSON
  is semantically equal.

Message definitions are authorable

The last of a mapping's four schema sources that a script could create — 74 of
327 mappings in the demo corpus (22.6%). Unlike an XML schema or a WSDL it holds
nothing external: it is a selection over the domain model.

create message definition collection Sales.MD_Order (
  definition OrderMessage for Sales.Order as 'Orders' (
    OrderId,
    Sales.OrderLine_Order/Sales.OrderLine as 'Lines' ( Sku ),
    Sales.Order_Customer/Sales.Customer ( FirstName )
  )
);